Page 106 - Intelligent Communication Systems

88 SNTELLIGENT COMMUNICATION SYSTEMS
This is different from the method that uses signal sequence or SDL. The char-
acteristics of this method are:
(1) The STR description is introduced.
(2) The calling process is simulated by the STR simulator.
Once a service is described in STR, the process is simulated immediately. It
is not necessary to describe all of the service procedures to simulate the calling
process. In this way rapid prototyping is performed with ease.

9.2 DEFINITIONS

Telecommunication services are described in terms of state and operation. For
example, plain old telephone service (POTS) is described as follows.
(1) Description of state: The idle state of the telephone is described as
idle(A)
This means that telephone A is in an idle state. The state in which a
dialtone is heard at telephone A is described as
dialtone(A)
In this way, a state is described by using the predicate.
(2) Description of operation: When telephone A calls telephone B, this is
defined as
dial(A, B)
To describe that telephone A is answered, we write
offhook(A)
(3) Description of telephone service: Here are some examples of telephone
service descriptions.
Examplel: idle(A) offhook(A): dialtone(A)
This means that when telephone A is idle and it is taken offhook, then a
dialtone is heard at telephone A.
Example 2: dialtone(A), idle(B) dial(A, B): ringback(A, B), ringing(B, A)
This means that a dialtone is heard at telephone A and telephone B is
idle. When telephone A calls telephone B, telephone B rings and a
ringback tone is heard at telephone A. This process is shown
schematically in Figure 9.3.
In general, a telephone onhook is the initial state. When the phone is taken
offhook or calls another phone, the phone gives a dialtone or a ringback tone. Thus,
when an event occurs at the phone, the state is transferred to the next state. The
transition of a state is shown in Figure 9.4.